The recent high cost of scrap as a feed material for steelmaking furnaces has caused steelmakers to turn elsewhere for their raw materials. One recently attractive raw material is reduced iron in the form of sponge iron, iron particles, pellets, briquets, and the like, which has been produced by the direct reduction of iron oxides or iron ores. Such materials will hereinafter be referred to collectively as metallized pellets. These metallized pellets are well suited as feed material, particularly to an electric arc steelmaking furnace. As a result, there have been a number of processes developed for their production. To be an attractive feed material, the pellets should be at least 85 percent reduced, and preferably over 90 percent reduced.
U.S. Pat. No. 3,375,099 discloses a direct reduction process in which iron ores are reduced in a shaft furnace by contact with hot reducing gases generated by the incomplete combustion of a mobile fuel, such as natural gas, with oxygen. The spent reducing gases, which are also known as top gases or off gases, are withdrawn from the shaft furnace, cooled and reintroduced at the bottom of the furnace as cooling gases to cool the product. The cooling gas is then allowed to flow upwardly through the shaft furnace, thus creating a closed circuit. It is also known that cooling of spent top gas enhances its reducing capacity. U.S. Pat. No. 3,748,120 teaches an improved method for reducing iron oxide to metallized iron, in which a reducing gas is catalytically reformed from a mixture of gaseous hydrocarbon and spent reducing gas from the reduction process. Cooling gas is circulated through the bottom portion or cooling zone of a shaft furnace in a closed loop, that is, the cooling gas is not allowed to flow upwardly into the reducing zone. U.S. Pat. No. 3,799,521 teaches that allowing cooling gas to flow upwardly into the reducing zone of a shaft furnace is detrimental in that it does not permit fully independent control of the reduction and cooling steps of the process. It further points out that to achieve a particular desired degree of carburization, the composition and flow-rate of the coolant gas should be controllable independently of the conditions existing in the reduction zone of the furnace.
When spent top gas is used for cooling the pellet product as shown in U.S. Pat. No. 3,375,099, the gas which flows upwardly into the reducing zone (upflow gas) is not fully preheated, because the flow of cooling gas in actual practice must exceed the theoretical quantity required. That is, the thermal capacity of upflowing gas must exceed the thermal capacity of the descending burden. This precludes the cooling gas being fully preheated by the hot burden. Hot, fresh reducing gas enters the reducing zone through bustle pipes around the perimeter of the furnace forcing the upwardly flowing cooling gas to the center of the furnace, which results in cooling the central portion of the burden in the reducing zone. Further, when the spent top gas is not upgraded prior to introduction as cooling gas, it has poor reducing potential when it enters the reduction zone. These two factors combine to cause incomplete reduction of the burden with a resulting lesser metallization of the product.